1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and, more particularly, to a method of providing a bonded structure of two compound semiconductor substrates.
2. Description of the Prior Art
A semiconductor device using a compound semiconductor such as GaAs has recently been attracting a great deal of attention in the fields of super high speed semiconductor devices and power semiconductor devices. In order to fully utilize advantages of such a compound semiconductor device, a technique for forming a junction between semiconductor layers having different impurities or conductivity types is required. Various types of epitaxial growth methods are known to form such a junction. However, it is very difficult to control the thickness and impurity concentration of an epitaxial layer within a wide range. For example, when a compound semiconductor device exhibiting a high breakdown voltage is manufactured, an epitaxial layer having a low impurity concentration and a considerable thickness must be formed in order to form a depletion layer and increase the breakdown voltage.
Liquid phase epitaxy is known as a technique for forming an epitaxial layer of a considerable thickness. In accordance with liquid phase epitaxy, it is comparatively easy to increase the thickness of an epitaxial layer. However, when a GaAs semiconductor device is formed, it is difficult to form an epitaxial layer having an impurity concentration of 10.sup.16 to 10.sup.17 /cm.sup.3 or lower. As a result, the avalanche voltage cannot be controlled because of the high impurity concentration, and a breakdown voltage of more than several tens of V cannot be obtained.
On the other hand, in accordance with vapor phase epitaxy, the impurity concentration of an epitaxial layer can be controlled to as low as about 10.sup.14 to 10.sup.15 /cm.sup.3. However, it is very difficult to form an epitaxial layer having a thickness of 10 to 20 .mu.m or more. If the thickness of an epitaxial layer is small, the punch-through voltage is low. Therefore, a breakdown voltage of only 200 to 300 V can be conventionally provided.
Meanwhile, a junction between different types of compound semiconductors having different band gaps can be applied as a heterojunction to various semiconductor elements. Various epitaxial growth methods are available for forming such a heterojunction. However, for epitaxial growth, in accordance with the epitaxial growth method, a semiconductor layer having a band gap different from that of an underlying semiconductor layer, the lattice constants of the semiconductor layers must basically match. Only a very, small number of combinations such as GaAs and AlGaAs can provide a good heterojunction. In many cases, crystal defects occur due to a strain resulting from lattice mismatch, or a barrier layer may undesirably form.
As another means for forming a heterojunction, a technique for bonding semiconductor substrates of different types by hot pressing or fusion bonding is known. However, the hot pressing requires high temperature and high pressure, and the fusion bonding is accompanied by fusion of the substrates. Therefore, many defects occur in the substrates and a thick interlayer is formed at the bonded interface, impairing electrical characteristics. When an impurity region is formed in one substrate, it is almost impossible to maintain its impurity concentration distribution after bonding, since a high temperature heating step is required.